88mm bore Ambassador

[v7john]

A friend of mine has what is said to be a 1969 750 Ambassador. At the weekend an inlet valve seat came loose and jammed the valve open so today I pulled the head off for him to go and get that sorted out.

The thing is it has steel-lined barrels with an 88mm bore (as used on the 950 models) and absolutely flat-topped pistons. No raised section on them at all. They don't look like Guzzi pistons to me. The bike started life in the USA and was imported to the UK over 6 years ago. My friend has owned it for 6 years. I should have measured what the stroke was but didn't think about it at the time.

Does this sound a familiar conversion to anyone here?

[Antietam Classic Cycle]

Kind of sounds like a "home brewed" big-bore kit using VW pistons.

[v7john]

Thanks Charlie. I've heard of this being done and wondered if it might be the case. Would this have possibly been done using the original short stroke crank from the 750? I might have to ride over there later in the week to measure the stroke. We haven't disturbed the barrel/piston.

[Furbo]

Thanks Charlie. I've heard of this being done and wondered if it might be the case. Would this have possibly been done using the original short stroke crank from the 750? I might have to ride over there later in the week to measure the stroke. We haven't disturbed the barrel/piston.

Quite probably yes. If you're going to rebuild the motor with VW pistons, you're probably not the type to spend $$ on a new crank!

Had a T3 engine with iron liners (but Guzzi pistons). Only issue was that one of the liners would twist and the rod would start knicking it. I'd dutifully pull the offending cylinder, remove a mm or 2 with a Dremel and replace it for another year.

[Antietam Classic Cycle]

What he said ^^^. I'd bet stock stroke, which will make it's displacement 851 cc.

[Turin]

851 means it's almost a ducati !

[Muzz]

851 means it's almost a ducati !

 

[1down5up]

Isn't the original crank a shorter stroke, and if so and it has 88mm bore cylinders from a later bike, won't it have an extremely low compression ratio?

(short stroke crank in long stroke barrels)?

[v7john]

Isn't the original crank a shorter stroke, and if so and it has 88mm bore cylinders from a later bike, won't it have an extremely low compression ratio?

(short stroke crank in long stroke barrels)?

I don't think there are short and long stroke barrels. The long stroke engines have shorter pistons so they don't stick out the top of the barrels. Deck height of the pistons remain the same so that wouldn't affect the compression. However it might have increased the compression ratio as the extra piston volume is being squashed into the original cylinder head capacity. Then again reduced by using a flat top piston instead of one with a crown/dome on it. Swings and roundabouts then.

[Antietam Classic Cycle]

When Guzzi built the 850 (844 cc actual) with it's longer 78 mm stroke, there was no change to the cylinders or connecting rods, only the pistons. The piston pin was located 4 mm closer to the crown.

If pistons meant for an engine with the 78 mm stroke were installed in an engine with 70 mm stroke, then yes, the result would be that the piston would be 4 mm down from the top of the cylinder at TDC and compression would be super low.